Acoustic Pannelling Part for a Vehicle

ABSTRACT

A cover part for a vehicle, especially an underbody cover, has a porous core layer and at least one cover layer on each side, wherein the porous core layer is constructed such that it has acoustic transparency or an acoustically absorbent effect. Here, the porous core layer is made either from a thermoplastic matrix with embedded reinforcement fibers, especially glass fibers, whose melting point temperature is higher than the melting point temperature of the plastic matrix, or from a foam, which is either open-cell or closed-cell and perforated. The acoustically absorbent porous core layer is occupied on one or both sides with one or more acoustically transparent or absorbent covering layers.

The invention relates to a cover part for a vehicle and especially to anengine compartment part or underbody cover part for a motor vehicle,which shall be used as the basis for examples below.

It is known to form underbody covers or engine compartment covers fromglass fiber-reinforced plastics in a forming method with high internalmold pressures. The glass-fiber reinforcement typically consists ofwoven mats or nonwoven mats, but also from bulk glass fibers, which are,however, as non-oriented as possible and which are introduced into aplastic matrix predominantly of polypropylene. The semifinished productsmade available in this way are usually plates made from glassfiber-reinforced thermoplastic (GMT) or rod-shaped granulate (LFT: longfiber thermoplastic). The rod-shaped granulate is composed of a glassfiber filament bundle of approximately 20 mm length, which is enclosedby a polypropylene shell. Before molding, the plates are heated in afurnace or the LFT granulate is melted in a plastifying unit, in orderto then be placed in the open mold of the press.

Meanwhile, it is also typical to process the glass fibers in a directdraw-in process together with plastic granulate in a plastifying unit(D-LFT), without having to pass through the intermediate step of an LFTsemifinished product. For increased temperature requirements, it is alsotypical to use as the plastic matrix a glass fiber-reinforcedduroplastic material made from polyester resin, which fully cures in aheated die (SMC: sheet molded compound).

The resulting components typically have a thickness of approximately1.5-2.5 mm and a basis weight of approximately 2 kg/m². The maximumcomponent size currently possible is approximately 1.0 to 1.5 m², due tothe very high molding pressures of approximately 200-300 bar and theassociated high machine costs for presses with a pressing force of morethan 3000 t.

New production methods allow the production of lighter-weight and largersurface-area components with significantly lower molding pressures.Here, a nonwoven mat made from glass fibers and plastic fibers, e.g.,polypropylene or polyester, is created as a semifinished product and iscovered on both sides with two plastic cover films, e.g., alsopolypropylene (LWRT: low weight reinforced thermoplastic). The corelayer of this composite has the property of expanding (lofting) underheating. With this material lofted to a total thickness of approximately10 mm, the edge region can be pressed compact (completely consolidated)through a suitable die shape, while the structure of the nonwoven corewith the cover films can be maintained in the remaining region. Thisstructure leads to very inherently stiff components with a relativelylow basis weight of below 1.5 kg/m². Because in this method the diecavity does not have to be formed by a flowing mass, significantlysmaller molding pressures (approximately 10 bar) are produced and it ispossible to mold with clamping surfaces of 4 m² and more withoutadditional measures. A disadvantage in this method is that structuresfor reinforcing or required for additional functionality, such asconnecting pieces, NACA openings, attachment domes, etc., can beattached only to a limited extent, if at all. Newer developments in thefield of LWRT have foam as the core layer and a glass fiber-reinforcedPP nonwoven as the cover layer. Here, further weight reduction ispossible for a comparable stiffness.

It is further known to provide these engine compartment shields andunderbody covers on the side facing the engine or the exhaustinstallation with heat shields and sound absorbers.

Sound absorbers usually consist of coated PUR foam or coated polyesternonwoven, but also of deep-drawn chamber structures or microperforatedfilms and plates. Typically, such sound absorption molded parts arelater bonded, clipped, or fused onto the engine compartment shielding.However, it is also known to produce a complete noise enclosure, thatis, a support and compartment absorber, in a blow-molding method in oneprocessing step. However, based on the process, this creates aconsiderable restriction on the material selection of the support andabsorber and thus also on the physical properties, especially asconcerns glass-fiber reinforcement of this component and its propertieswith reference to stiffness, strength, and impact resistance.

Heat shields are composed of prefabricated aluminum, which is clipped onor fused on by means of a special connection layer. The fusing andshaping of fusible aluminum in the die is also known.

Recently, the combination of sound absorption and heat insulation in theform of aluminum membrane absorbers and microperforated aluminum filmshas also become known.

It is further known to produce wheel-well covers from nonwovens orcombinations of nonwovens and films. Nonwoven variants have advantagesin terms of production costs and component weight compared withinjection-molded wheel well covers. In particular, it has been shownthat this nonwoven has a favorable acoustic effect against noises fromsplashed water and stone impacts.

Recently, production has moved towards attaching nonwoven also on theroad-surface side of the underbody covers and noise enclosure. Here, ithas been shown that the noise from the engine, gearing, and exhaustinstallation is reduced by this street-side lining and then even whenthe underbody has already been closed completely by a noise enclosure.To take full advantage of the potential, the nonwoven thickness shouldbe significantly greater than the current typical nonwoven thicknesslayers of approximately 1 mm.

The invention described here is based on the task of creating a coverpart for a vehicle, especially for the underbody, cover noise enclosurecover, or wheel well cover that integrates the properties of the systemsdescribed above for acoustic and heat insulation while maintaining thestiffness, impact resistance, and elastic modulus properties of theglass fiber-reinforced components.

According to the invention, the components of glass fiber-reinforcedsupport plates, sound absorbers, heat insulators, nonwoven linings,etc., are no longer separated in their functional arrangement and intheir successive production. The properties are combined in a singlematerial or layer arrangement, wherein in a single shaping process anengine compartment cover, an underbody cover, a wheel well, or otherinherently stiff components are produced from such a material composite.This results in clear cost reductions compared with most well knownmultiple-step production methods. Finally, with respect to currentdevelopments for always using increasingly larger surface-areacomponents in the underbody region, an advantageous method requires nocompromises in this respect and allows, for example, the production of acompletely closed underbody assembly made from a single large plate.

The method according to the invention offers the prerequisites forcreating components having all of the properties named above, whereinboth the production is simplified and thus less expensive and also thefunctional properties can be maintained or even increased.

The basic idea of the invention is to combine a porous core layer withacoustically transparent or absorbent cover layers such that both themechanical properties of classic support materials and also the acousticproperties of classic absorbers are realized. Thus, the support itselfbecomes an absorber and contributes with its material strength to theeffective acoustic total thickness of the component. However, theeffective acoustic surface is also increased, because now the areas thatwere not previously equipped with additional absorbers due toinstallation space are now also acoustically active. Additionalabsorbers to be attached at a later time are no longer necessary.

The low-pressure molding process necessary for processing porousmaterials has the result that all of the material components can beshaped and connected in one step.

The technical aspect concerning the method lies especially in thatthrough the low-pressure process in the LWRT production, the effectiveacoustic and thermal layers are shaped and connected together with theLWRT core in a one-step process.

Advantages compared with the state of the art are:

-   No separate shaping and stamping of the support necessary;-   No separate shaping and stamping of the absorber or heat-protection    aluminum film necessary;-   No separate connection of the absorber or heat-protection aluminum    film with the support necessary.-   A nonwoven pressed at the same time maintains to a large degree its    original thickness and thus its acoustic performance.

One aspect concerning the component lies in that, through suitableselection and shaping of the cover layers of the LWRT core layer (glassfiber PP core layer or the porous foam layer), the core layer is usedsimultaneously as an effective acoustic air volume and thus theeffective acoustic total thickness of the component grows by the corelayer thickness.

Open-celled, porous materials, such as foams and nonwovens, areacoustically absorbent if their flow resistance assumes certainparameters. For nonwovens, the setting of this flow resistance istypically realized by suitable compaction of the fibers. The PP glassfiber mixture of an LWRT core can also be compressed suitably and thusallows this acoustic adjustability, wherein the reinforcing propertiesof the bound glass-fiber structure are maintained.

According to the invention, the cover layers of the porous glass fiberPP core layer or the porous foam layer have an acoustic transparency oreven inherent absorbent effect. Furthermore, the core layer can beadjusted in its acoustic effect through suitable dimensioning of thefibers and the fiber density or the foam structure, wherein the flowresistance of the core layer determines the acoustic tuning of thislayer. The dimensioning of the longitudinal flow resistance Ξ at a givenlayer thickness and required lower limit frequency is described in“Technical Noise Protection” by Werner Schirmer (VDI Verlag ISBN3-540-62128-8). There, it is recommended that the optimum adaptationoccurs at Ξ_(opt)d=800 to 2400 Ns/m³. However, this is not absolutelynecessary for a good acoustic design of the total system, because thetotal acoustics can also be decisively influenced by means of a gooddesign of the cover layers.

An open-celled, porous absorber has a nearly straight-line increase inthe sound absorption from 0 to 100% for an optimum flow resistance(Ξ_(opt)d=800 to 2400 Ns/m³) with increasing frequency, in order toblock oscillations at a level close to 100% for an additional increasein frequency. The relationship between the thickness d of the porousabsorber and the first frequency of the 100% maximum is approximatelygiven by the relationship f=c*N/(4*d); N=1, 3, 5 (c: speed ofpropagation in air). This follows from the fact that a porous absorberhas absorption maximum values, where an oscillation at a ¼, ¾, 5/4wavelength, etc., passes into the absorber or, in other words, where thesound particle velocity has an antinode at the absorber surface.

However, closed-pore foams also exhibit acoustic absorption propertieswhen the pores reach a certain size and the cell walls are elastic. Inthis case, the core acts acoustically as a series circuit of smallmembranes. Also, closed-cell foam can be perforated or pierced byneedles, which contributes to a further increase in the sound absorptionproperty. Useful material thickness values for the core layer liebetween 1 mm and 20 mm and especially between 1.5 mm and 10 mm,independent of whether a nonwoven or a foam material is used as the corelayer.

The acoustic transparency of the cover layers is achieved, on the onehand, through perforations. In the case of hole area ratios >30%,transparency is achieved to a large extent. For hole area ratios under10%, the film obtains inherent damping properties and thus absorptionproperties, if the hole sizes here lie between 0.01 and 1 mm, preferablybetween 0.05 and 0.2 mm.

On the other hand, if one uses a nonwoven as the cover layer, then thetransparency or absorbent property can also be set by means of itslongitudinal flow resistance Ξ.

Finally, acoustic transparency can also be realized by a thin film,wherein this film might not be integrated rigidly into the corestructure. One solution is to use foam film, like that also used incommon chamber absorbers or to bind the cover film only partially to thecore through the formation of chambers in the shaping process (by meansof vacuum forming or blow molding). In the case of the foam film,commercially available PP foams can be used, e.g., Alveolen NPFRG 2905.5made by Alveo or Procell-P 150-2.5 SF40 made by Polymer-Tec. In the caseof compact films, common films of 0.1-0.8 mm are suitable if the bond tothe core is broken through the chamber formation or films <100 μm if thefilm is in a direct bond to the core material. In the case of a coverwith films or foams, a resonance absorber is produced whose resonancefrequency is calculated approximately from the damped air stiffness andthe mass of the cover film with fres=1/2PI* (surface stiffness/surfacemass) with surface stiffness=rho*c²/d (with rho=air density; c=speed ofsound; d=thickness of the film). Here, one also sees that the thicknessof the air layer determines for the lower limit frequency.

The material thickness of the cover layers for compact films should liebetween 20 μm and 500 μm and especially between 20 μm and 100 μm, forfoam films between 1 mm and 8 mm and especially between 2 mm and 6 mm,and for nonwovens it should lie between 0.5 mm and 5 mm and especiallybetween 1 mm and 3 mm.

For at least one cover layer or for both cover layers, a fiber nonwovenreinforced with fibers, especially with glass fibers, can be used, andis preferably composed of 60 wt % to 80 wt % thermoplastic fibers, forexample, PP fibers, and from 20 wt % to 40 wt % reinforcement fibers,for example, glass fibers. In the preferred construction, the nonwovenof the cover layer is constructed with a basis weight of 400 g/m² to 500g/m² from approximately 75 wt % PP fibers and 25 wt % glass fibers.

To prevent the glass fibers from projecting past the outside, anotherthin PET-fiber nonwoven cover with a basis weight of 15 g/m² to 50 g/m²can be arranged on the cover layer or cover layers.

In this connection, a fiber nonwoven with approximately 60 wt % PPfibers and approximately 40 wt % glass fibers has proven effective asthe core layer, which has a basis weight of 400 g/m² to 1200 g/m² andespecially of 500 g/m² to 700 g/m².

The production of a component is realized preferably in that thesemifinished product, which consists of one or more layers of films,foams, and nonwovens, is heated in a contact or radiant heater and thenmolded in a cooling die. Layers not subjected to the heating process,for example, the covering nonwovens or the aluminum films for radiantheating, are introduced directly into the die and molded together withthe heated core material. It is also conceivable to feed all of thelayers separately and to connect them while suitably preheated for thefirst time in the die. Thus, the creation of a semifinished product canbe eliminated, which in turn results in cost savings.

In a preferred construction of the invention, for example, an LWRTsemifinished product, which is made from a reinforced glass-fiber PPcore nonwoven and covered with two stabilizing films, is modified sothat, on the one hand, the glass fiber PP core nonwoven developsacoustic potential in its structure and the cover layers achievefunctionality that goes beyond the pure covering and reinforcingfunctions.

For such an underbody cover made from a porous core layer and at leastone cover layer on each side, the cover film can vibrate like amembrane, for example, everywhere or in some regions, wherein themembrane-like vibration of the cover film can be achieved throughmaterial selection of an especially elastic film. Plastic films madefrom PP, PET, PA, PU, etc., with a thickness<100 μm are in this sensefundamentally elastic without requiring special softening additives.

For an inherently stable, acoustically absorbent component or moldedpart, the membrane-like vibration can be achieved in that the bonding tothe core material is broken in some areas. This can be realized, forexample, in that the film or foam film is drawn with vacuum into chambercavities of the die before the die halves are brought together or ispressed away from the core layer again by a vacuum or compressed airafter the halves are brought together.

In the method for producing an inherently stable, acoustically absorbentunderbody cover, the covering layers can be made from high-melting-pointand low-melting-point fibers and thus the fiber structure of thehigh-melting-point fibers is maintained despite the heating, while thelow-melting-point fibers are used as bonding fibers. Alternatively,instead of the fibers, composite films made from high-melting-point andlow-melting-point films can also be used.

In the method for producing an inherently stable, acoustically absorbentunderbody cover, as the covering layer, a high-melting-point film, e.g.,an aluminum film, with a core material-side low-melting-pointthermoplastic layer or bonding agent layer can be brought into the dieand molded together with the heated core material and in this way, theadhesive layer is activated, which finally leads to a bond between thecore material and the high-melting-point film.

Instead of aluminum, high-melting-point plastic films made from PA, PET,and PUR can be used. Examples are: polyamide film 20-50 μm, polyesterfilm 20-50 μm, polyurethane film 20-50 μm, each with a thin one-sided ortwo-sided adhesive or thermoplastic layer, for example, made frompolypropylene, which can be used as a low-melting-point bonding layer.The polypropylene melts while the actual cover film survives the heatingor sealing without melting. This is especially advantageous for the useof microperforated films, because, according to experience, the holesizes change during heating.

In general, the cover layers can be made both from high andlow-melting-point fiber mixtures and also from high- andlow-melting-point film combinations, wherein for the films, thelow-melting-point layers can also be applied to both sides if anotherouter layer is to be pressed on simultaneously.

According to the invention, the covering layers can be applied partiallyonly in the regions of the component, where they are required forfunctional reasons. The partial application of the covering layers canbe realized in that the covering layers are connected to the corematerial only in some regions. In one possible construction, thecovering film is connected to the core material only in some regionsthrough partial heating, partial ultrasonic welding, partial highfrequency welding, or partial friction welding. Alternatively, it isalso possible for the covering film, which has a high-melting-pointlayer and a core-side low-melting-point layer, to be heated to atemperature that causes only the low-melting-point layer to melt, andthen to be pressed onto the core from the side facing away from the corematerial and in this way to be partially attached to this core. Thelow-melting-point layer can here extend past the entirehigh-melting-point layer, but it is also possible for the core-side,low-melting-point layer to be arranged in only some regions of thehigh-melting-point layer.

For the inherently stable, acoustically absorbent underbody cover, thecore material can be made from PP foam film or from PUR foam. PP foam istypically closed-cell, but could also be modified by perforations or theapplication of stamps, such that a comparable effect to the fiber corelayers is also produced. The perforation parameters should be selectedsimilar to the microperforated cover layers, such that the hole diameteror the slit width lies between 0.01 mm and 1 mm, preferably between 0.05mm and 0.2 mm, and the ratio of hole area to total area lies in therange between 0.1% and 10%, preferably between 3% and 8%. Surprisingly,it has been shown that even for only surface perforations, i.e., withoutpiercing the layer, a considerable acoustic effect is produced. Thestamps and the intermediate connecting pieces should move in the rangefrom 5 to 50 mm with round, square, or honeycomb geometry. Anotheradvantage is weight reduction.

In the enclosed figures, embodiments of the invention are described.Shown are:

FIGS. 1 a, 1 b, 1 c, known constructions of a cover part,

FIGS. 2 a, 2 b, 2 c, constructions according to the invention for acover part,

FIGS. 3 a, 3 b, 3 c, 3 d, 3 e, 3 f, additional constructions accordingto the invention for a cover part, and

FIG. 4, a schematic representation of the method for producing a coverpart.

FIGS. 1 a, 1 b, and 1 c show various known noise enclosure systemscurrently in use made from a cover plate made from, e.g., GMT, D-LFT, orLWRT, and additionally installed sound absorbers.

In FIG. 1 a, a porous absorber 2 made from, for example, a polyesternonwoven or a PUR foam is shown schematically, which is covered with athin PUR or polyester film 1 a against fluid media and is applied on asupport 3 made from, for example, GMT.

In FIG. 1 b, a porous absorber 2 made from, for example, basalt stonewool, is shown schematically, which is covered with a microperforatedaluminum film 1 b and which is applied on a support 3 made from, forexample, SMC.

In FIG. 1 c, a chamber absorber 10 made from PP foam is shownschematically, which is applied on a support 3 made from, for example,LWRT.

FIGS. 2 a, 2 b, and 2 c show embodiments according to the inventionanalogous to FIGS. 1 a, 1 b, and 1 c, in which the functionality of theacoustics were transferred into the LWRT core.

In FIG. 2 a, an acoustically active glass fiber PP core layer 4 isenclosed between two cover layers 5 a and 5 b made from film. The corelayer 4 with a basis weight of 1200 g/m² has a thickness of 5 mm and iscomposed of 40 weight percent glass fiber with 15-20 μm fiber diameterand 60 weight percent PP, which is melted and binds the glass fibers.The films have a more or less pronounced acoustic function according tothe bending properties and bonding to the glass fiber PP core 4.

In FIG. 2 b, the acoustically active glass fiber PP core layer 4 iscovered on the top side with a 100 μm thick microperforated film, forexample, made from aluminum 5 c. This is acoustically absorbent with,for example, a hole diameter of 100 μm and a hole spacing of 500 μm. Thealuminum material allows this cover part also to be attached in theimmediate vicinity of the exhaust line of the vehicle and thus alsoallows a closed underbody.

In FIG. 2 c, the total construction of the cover films 5 a and 5 b andthe glass-fiber PP core layer 4 is molded into a chamber structure 6.The chamber side surfaces form a square with side lengths from 10 to 100mm and the height of the chamber lies in the range from 5 to 30 mm,wherein the ratio of side length to height should be approximately 1 to2. The chambers expand the acoustic adjustability of the component bymeans of the chamber geometry (resonator effect) and also allowadditional stiffening or relaxing of the entire component according tothe geometry and arrangement of the chambers.

Additional embodiments of the invention are shown in FIGS. 3 a to 3 f.

In FIG. 3 a, the LWRT core layer 4 is covered with a foam film 5 d,e.g., Alveolen NPFRG 2905.5 made by Alveo or Procell-P 150-2.5 SF40 madeby Polymer-Tec.

FIG. 3 b shows the same material construction, wherein here additionalchambers 6 are inserted into the component.

In FIG. 3 c, the bottom side of the component is covered with a film 5 band a 2 mm thick acoustically active PP nonwoven 5 e with a basis weightof 500 g/m². The top side 5 a has a cover made from aluminum film 5 cfor thermal shielding against the temperatures of the exhaust line.

In FIG. 3 d, the bottom side of the component is also covered with afilm 5 b and an acoustically active nonwoven 5 e. The 0.05 mm thickaluminum film Sc of the top side is microperforated with a hole diameterof 0.2 mm and a hole spacing of 1.5 mm, which significantly increasesthe acoustic effect of the component even more.

In FIG. 3 e, the top side and the bottom side of the component arecovered with a microperforated film 5 f.

In FIG. 3 f, the top side and the bottom side of the component arecovered with a 0.05 mm thick microperforated film 5 f with a holediameter of 0.2 mm and a hole spacing of 1.5 mm and these are eachcovered again with an acoustically active nonwoven 5 g. This embodimenthas the advantage that the nonwovens 5 f are active at a high frequencyand the core layer can be tuned in medium and low frequencies togetherwith the microperforated covers. With suitable oleophobic andhydrophobic construction of the nonwovens, this has the effect that,e.g., splashed water cannot penetrate into the perforations and thusinto the core material.

Naturally, other embodiments are also conceivable and useful accordingto the requirements on the acoustic and thermal function of thecomponent. In particular, it is also useful to provide cover layers ortheir structuring in chambers, etc., only partially.

FIG. 4 shows the production method, wherein the core layer 4 withpossible cover films 5 a, 5 b is heated in a radiant heater or contactheater 7 and in this way the core layer is lofted. This lofted materialis then inserted and molded in a press die 8 with other non-preheatedcover layers 5 c and 5 d.

Finally, the goal is to build components with the largest possiblesurface area, because on the one hand, the absorption increasesproportionally with the surface area, and, on the other hand, the degreeof closure of the underbody increases due to the large surface area, andthus the output of sound is also prevented. Both effects lead toover-proportioned improvement of the outside noise. Such underbodycovers can be produced economically with the described component conceptand the preceding production method, with which novel possibilities inthe acoustic and aerodynamic shaping of vehicles is produced.

1-25. (canceled)
 26. A method for producing an inherently stable,acoustically absorbent cover part for a vehicle, especially an underbodycover, comprising: heating an acoustically absorbent core material; andshaping the heated core material together with one or more plasticcovering layers in a mold and pressing the core material and thecovering layers together.
 27. The method according to claim 26, whereinthe covering layers are heated with the core material and then moldedand pressed together with the core material.
 28. The method according toclaim 27, wherein the covering layers are made from high-melting-pointfibers and low-melting-point fibers and thus the fiber structure of thehigh-melting-point fibers is maintained despite the heating, while thelow-melting-point fibers are used as bonding fibers.
 29. The methodaccording to claim 26, wherein the covering layers are inserted into themold without preheating and are molded and pressed together with theheated core material.
 30. The method according to claim 26, wherein atleast one of the covering layers comprises an aluminum film with a corematerial-side bonding agent layer; and wherein inserting said coveringlayer into the mold with the heated core material activates the adhesivebonding agent layer which leads to a bond between the core material andthe covering layer.
 31. The method according to claim 30, wherein thealuminum film is microperforated.
 32. The method according to claim 26,wherein at least one of the covering layers comprises ahigh-melting-point plastic film with a core material-side bonding agentlayer; and wherein inserting said covering layer into the mold with theheated core material activates the bonding agent layer, which leads to abond between the core material and the covering layer.
 33. The methodaccording to claim 32, wherein the plastic film is microperforated. 34.The method according to claim 26, wherein the core material and coveringlayers are vacuum shaped in the mold.
 35. The method according to claim26, further comprising the step of introducing compressed air betweenthe covering layers and the core material.
 36. The method according toclaim 26 wherein the core material comprises a nonwoven made from athermoplastic matrix and reinforcement fibers and wherein thereinforcement fibers have a higher melting point temperature than thethermoplastic matrix.
 37. The method according to claim 36 wherein thereinforcement fibers of the core material are made from polyester (PET)or from polyamide (PA) or from glass.
 38. The method according to claim36 wherein the thermoplastic matrix is made fromlow-melting-point-fibers.
 39. The method according to claim 38, whereinthe low-melting-point fibers are made from polypropylene (PP).
 40. Themethod according to claim 38, wherein the low-melting-point fibers aremade from polyester (PET).
 41. The method according to claim 38, whereinthe low-melting-point fibers are made from polyamide (PA).
 42. Themethod according to claim 26, wherein the heating is realized by aradiant heater.
 43. The method according to claim 26, wherein theheating is realized by means of a contact heater.
 44. The methodaccording to claim 26, wherein the covering layers are applied only inselected regions of at least one side of the core material.
 45. Themethod according to claim 26, wherein the covering layers are connectedto the core material only in some regions of the core material.
 46. Themethod according to claim 45, wherein at least one of the coveringlayers is connected to the core material only in some regions of thecore material through partial heating, partial ultrasonic welding,partial high frequency welding, or partial friction welding.
 47. Themethod according to claim 45, wherein at least one of the coveringlayers comprises a high-melting-point layer and a core-sidelow-melting-point layer; and wherein said covering layer, is heated to atemperature that melts only the low-melting-point layer, and then ispressed to the core material such that the low-melting-point layercontacts the core material.
 48. The method according to claim 46,wherein the core-side, low-melting-point layer is arranged only in someregions of the high-melting-point layer.
 49. The method according toclaim 26, wherein the core material is made from PP foam film.
 50. Themethod according to claim 26, wherein the core material is made from PURfoam.
 51. The method according to claim 30, wherein the bonding agentlayer comprises a low-melting-point thermoplastic.
 52. The methodaccording to claim 28, wherein the high-melting-point fibers of thecovering layers are made from polyester (PET) or from polyamide (PA) orfrom glass.